We have expressed in E. coli segments of the rod portion of rabbit skeletal fast muscle myosin and compared physical properties of two different species, LMM-30 and LMM-30C'. LMM-30 consists of 263 amino acids including the original C-terminus of myosin heavy chain. LMM-30C' is colinear with LMM-30, but is devoid of 17 residues at the C-terminus. 1H NMR spectroscopy indicates that the C-terminus of LMM-30, but not of LMM-30C' is unfolded and freely mobile. Furthermore, the present results show that the unfolded C-terminus is essential for molecular assembly of LMM-30; at pH 8.0 LMM-30, but not LMM-30C', formed aggregates upon decreasing the ionic strength.